1. Scope of the Invention
This invention relates generally to conductivity meters and more particularly to a portable conductivity meter for automatically accurately and repeatably measuring the conductivity of an electrolyte used in artificial kidney machines.
2. Description of the Prior Art
Conductivitity meters for measuring the electrical conductivity of electrolytic dialysate solutions for artificial kidney machines are known in the art. Most prior art conductivity meters are stationary apparatus having a conductivity cell or a pair of electrodes, permanently affixed to a housing submersed within an electrolyte, and electrically connected with a stationary control apparatus or other piece of peripheral equipment for providing an output control signal responsive to the conductivity cell or electrode measurement. The conductivity meters, while designed to adequately measure the electrolyte conductivity within that particular electrolyte container with which they are used, generally do not provide accurate and repeatable conductivity measurements without recalibration if used with electrolyte containers having configurations different from that for which the meter was previously calibrated. Similarly, recalibration of the meter will be required if the position of the electrodes relative the walls of the same electrolyte container is changed. These results are due to the fact that conductivity measurements made via electrode pairs are inherently sensitive to the size and shape of the electric field developed therebetween, (i.e. to the "effective volume" of electrolyte sample seen by the electrodes). The size and shape of the electric field developed between the electrodes necessarily varies as a function of the size and configuration of the electrolyte container and with the relative positioning of the electrodes therein.
A portable conductivity meter overcomes many of the disadvantages of the stationary single purpose conductivity meters and is intended to provide rapid mobile conductivity measurement of any portion of a larger electrolyte sample. Such portable conductivity meters are required in many applications wherein a stationary conductivity meter is not required, to check the accuracy of a stationary conductivity meter permanently implanted within the electrolyte, to provide a rapid accurate conductivity reading of the electrolyte for qualifications or test purposes, or the like. While providing increased flexibility of use advantages, prior art portable conductivity meters which have expanded scales required for accurately measuring conductivity over a narrow conductivity range, have generally been expensive and have not displayed the stability or repeatability required for such applications.
While accuracy and repeatability can oftentimes be sacrificed, there are certain applications wherein there is absolutely no room for error in either the accuracy or repeatability of a conductivity measurement. One such application is in the conductivity measurement of a dialysate electrolyte employed for use within an artificial kidney system. In such systems, the conductivity of the dialysate provides an accurate indication of the ionized salts (such as sodium chloride) present within that dialysate. Such salt concentrations must be accurately known within an artificial kidney system to provide proper operation of the artificial kidney machine in the hemodialysis process, on an individual patient requirement basis. Since the required dialysate conductivity for an artificial kidney system can differ from patient to patient, it becomes extremely important to have a portable conductivity meter available which can accurately and rapidly measure the dialysate conductivity, both in batch type and in proportioning type artificial kidney systems.
The primary disadvantages of prior art portable conductivity meters used for measuring the conductivity of dialysate electrolytes, have been their high cost inaccuracy, instability and nonrepeatability. One such prior art dialysate conductivity meter comprises a single hand-held chassis housing conductivity measurement circuits, having a conductivity indicator on an upper surface and a pair of electrodes projecting from its lower surface. This device only requires the operator to physically hold the chassis above the electrolyte sample such that the electrodes are immersed therein to effect a conductivity measurement, but suffers inaccuracies and nonrepeatability in measurement due to operator variances in positioning of the electrodes relative the walls of the electrolyte container, as above described with respect to the stationary conductivity meters.
Another prior art portable dialysate conductivity meter attempts to cure the inaccuracy and nonrepeatability problems of the above-described meter by defining a fixed measurement volume within the chassis of the meter, but requires a plurality of steps to be performed for making a measurement. This device employs a sample scoop member with which a sample of electrolyte is physically removed (scooped) from a larger batch thereof, and requires the operator to physically deposit the scooped electrolyte sampe into a chassis of the conductivity meter. The scoop member is designed such that its handle portion can be thereafter inserted within the conductivity meter chassis in such a manner so as to cause a thin volume of the scooped electrolyte sample to be formed between and to physically engage a pair of measurement electrodes, after which the actual conductivity measurement can be made. The conductivity measurement is made on the thus isolated electrolyte sample by passing a DC current between the electrodes and through the electrolyte sample. The DC current, however, electrolyzes the dialysate sufficiently so as to distort the conductivity reading. Therefore, besides requiring the operator to perform a plurality of measurement steps, this prior art device introduces inaccuracies into the measurement itself, casuing nonrepeatability in measurements.
The present invention overcomes the above disadvantages of both the stationary and the portable conductivity meters of the prior art, and offers a portable conductivity meter operable with rapid one-step operation to automatically provide an accurate dialysate conductivity measurement over a narrow conductivity range which is reliably repeatable over time.